The number of important intermediates in the chemical industry which contain a biaryl structure has increased greatly in recent years. Owing to their applications in the pharmaceutical and agricultural sectors, producers of such intermediates are concerned not only with price, but also with the high purity requirements. For these reasons, highly active, stable and highly selective catalyst systems for the C,C couplings mostly employed for preparing unsymmetrical biaryls are sought. Particularly in the case of couplings of nonactivated aromatics, especially chloroaromatics, the catalyst systems known hitherto generally require the use of large amounts of catalyst (up to 5 mol % or more) in order to achieve industrially useful conversions. Nevertheless, the compositions of the reaction mixtures obtained are often so complex that simple recycling of the catalyst is no longer possible and as a result the high catalysts costs stand in the way of industrial implementation.
It is therefore an object of the present invention to prepare biaryls in high yields, high selectivity and high purity and to use catalyst systems which can be obtained simply and inexpensively and are stable on storage, lead to a high TON (turnover number) and TOF (turnover frequency) and have long operating lives.
It has surprisingly been found that palladaphosphacyclobutanes meet the abovementioned requirements and even in very small amounts make Suzuki couplings, even of chloroaromatics, possible under gentle conditions. The reaction products are obtained in high yields and in high purity even after only simple and inexpensive purification steps. Astonishingly, the palladaphosphacyclobutanes used at the same time have very high activity and high stability so that it is possible to use very small amounts of catalyst. The small amounts of catalyst result in economic and ecological advantages since waste products and waste-intensive work-up processes are avoided. The process of the invention thus meets the demands which are made of a process which can readily be implemented in industry.
The present invention provides a process for preparing biaryls of the formula (1), 
where
R1 to R10 are identical or different and are each, independently of one another, hydrogen, straight-chain or branched alkyl having from 1 to 8 carbon atoms, cycloalkyl which has from 3 to 7 carbon atoms in the ring and may bear C1-C4-alkyl(s) as substituent(s), C2-C8-alkenyl, C2-C8-alkynyl, fluorine, chlorine, hydroxy, OLi, ONa, OK, OMg0.5, OMgCl, OMgBr, alkoxy having from 1 to 8 carbon atoms, NH2, NHRxe2x80x2, NRxe2x80x22, NH(Cxe2x95x90O)Rxe2x80x2, NH(Cxe2x95x90O)ORxe2x80x2, NH(Cxe2x95x90O)NRxe2x80x22, NO2, SO2Rxe2x80x2, SORxe2x80x2, POphenyl2, POxe2x80x94(C1-C8-alkyl)2, PO3xe2x80x94(C1-C8-alkyl)2, (Cxe2x95x90O)Rxe2x80x2, C(xe2x95x90O)NRxe2x80x22, C(xe2x95x90O)ORxe2x80x2, CN, CO2Li, CO2Na, CO2K, CO2MgCl, CO2MgBr, phenyl, substituted phenyl, aralkyl or heteroaryl; or two adjacent radicals R(n) and R(n+1) correspond to a bridging 1,xcfx89-alkanediyl chain having from 3 to 8 carbon atoms or a bridging ethylene dioxy or methylene dioxy chain; or two adjacent radicals R(n) and R(n+1) correspond to a unit of the formula 
xe2x80x83and the radicals Rxe2x80x2 are, independently of one another, hydrogen, C1-C8-alkyl, C1-C8-alkoxy or phenyl, and the ring atoms X1 to X10 are either all carbon atoms (biphenyls) or at most one heteroatom is present in each of the two linked rings A and B such that any ring member RiXi is N (phenylpyridines, bipyridines), or two adjacent ring members R2X2 and R3X3, or R4X4 and R5X5, or R7X7 and R8X8, or R9X9 and R10X10, are replaced by S, O or NRxe2x80x3 (e.g. phenylthiophenes, phenylpyrroles, phenylfurans, pyridylfurans, pyridylpyrroles), where Rxe2x80x3 is hydrogen, alkyl having from 1 to 8 carbon atoms, SiRxe2x80x23 or C(xe2x95x90O)Rxe2x80x2,
by coupling aromatics of the formula (2) with an aromatic boron compound of the formula (3), 
where LG is one of the leaving groups fluorine, chlorine, bromine, iodine, triflate, perfluoro-(C1-C8)alkylsulfonate, mesylate, tosylate, nosylate (p-nitrophenylsulfonate), brombenzenesulfonate or N(OSO2CF3)2;
Q1 and Q2 are identical or different and are each OH or a radical of the formula xe2x80x94Oxe2x80x94(C1-C8)alkyl, xe2x80x94Oxe2x80x94(C2-C8)-alkenyl, xe2x80x94Oxe2x80x94(C2-C8)alkynyl, xe2x80x94O-aryl or xe2x80x94O-alkylaryl, or Q1, Q2 and the adjacent boron atom form a cyclic boric ester of the alcohols (C3-C12)-cycloalkane-1,2-diol, (C5-C12)-cycloalkene-1,2-diol, (C5-C12)-cycloalkane-1,3-diol, (C5-C12)-cycloalkene-1,3-diol or of alcohols of the formulae (Va) to (Ve), 
where R1b to R8b are identical or different and are each, independently of one another, hydrogen, C1-C12-alkyl, C1-C12-hydroxyalkyl, C2-C12-alkenyl, C2-C12-alkynyl, C3-C12-cycloalkyl, (C1-C12)-alkoxy, (C1-C12)-acyloxy, O-phenyl, O-benzyl, aryl, heteroaryl, fluorine, chlorine, bromine, iodine, NO2, NH2, N(alkyl)2, N[Si(C1-C4-alkyl)3]2, CF3, CCl3 or CBr3,
and/or two adjacent radicals R1b to R8b together form a 5- to 8-membered aliphatic or aromatic ring, e.g. a phenyl ring, and n is an integer from 2 to 12,
or Q1 and Q2 together form a divalent radical of the formula (Vf) 
wherein the coupling is carried out in the presence of a palladium compound of the formula (IV), 
xe2x80x83where R1a and R2a are each, independently of one another hydrogen, (C1-C4)-alkyl, (C3-C12)-cycloalkyl, (C1-C4)-alkoxy, fluorine, N(C1-C4-alkyl)2, CO2xe2x80x94(C1-C4-alkyl), OCOxe2x80x94(C1-C4)-alkyl or substituted or unsubstituted aryl;
xe2x80x83R3a, R4a, R5a and R6a are, independently of one another, (C1-C8)-alkyl, (C3-C8)-cycloalkyl, substituted or unsubstituted aryl;
xe2x80x83or R1a and R2a, or R2a and R3a, or R3a and R4a, together form an aliphatic ring having from 4 to 10 carbon atoms,
xe2x80x83or R5a and R6a together with the P atom form a saturated or unsaturated 4- to 9-membered ring,
xe2x80x83or R4a and R5a form a bridging 1,xcfx89-alkanediyl chain having from 2 to 7 carbon atoms;
xe2x80x83and
xe2x80x83Y is an anion of an inorganic or organic acid, a xcex1,xcex3-diketo compound or a 5- or 6-membered nitrogen-containing heterocycle,
and in the presence of a base and a solvent at temperatures of from 20xc2x0 C. to 200xc2x0 C.
The process of the invention makes it possible to prepare, for example, biphenyls, phenylpyridines, phenylfurans, phenylpyrroles, phenylthiophenes, bipyridines, pyridylfurans and pyridylpyrroles.
Preference is given to compounds of the formula (1) in which
R1 to R10 are identical or different and are each hydrogen, straight-chain or branched C1-C4-alkyl, C5-C6-cycloalkyl, methyl-(C5-C6)cycloalkyl, C2C4-alkenyl, C2-C4-alkynyl, fluorine, chlorine, hydroxy, C2-C4-alkoxy, NH2, NHRxe2x80x2, NRxe2x80x22, NHCORxe2x80x2, NHCOORxe2x80x2, COOH, COORxe2x80x2, CN, phenyl, a phenyl, benzyl or pyridyl substituted by from 1 to 3 radicals selected from the group consisting of C1-C4-alkyl, F, Cl, C2-C4-alkoxy or NO2, or two adjacent radicals R(n) and R(n+1) form a 1,xcfx89-alkyldiyl chain having from 4 to 6 carbon atoms, and
Rxe2x80x2 is hydrogen, C1-C4-alkyl, C1-C6-alkoxy or phenyl.
Preferred aromatic boron compounds of the formula (3) are ones in which
R6 to R10 are as defined above and
Q1 and Q2 are each a radical of the formula OH, xe2x80x94Oxe2x80x94(C1-C4)-alkyl, xe2x80x94Oxe2x80x94(C2-C4)-alkenyl, xe2x80x94Oxe2x80x94(C2-C4)-alkynyl, O-phenyl or xe2x80x94O-benzyl, or
Q1, Q2 and the adjacent boron atom form a cyclic boric ester of the alcohols ethylene glycol, 1,3-propanediol, 1,4-butanediol, 2,2-dimethylpropane-1,3-diol, catechol, pinacol, 2,3-dihydroxynaphthalene, diethanolamine, triethanolamine, 1,2-dihydroxycyclohexane, 1,3-dihydroxycyclopentane or 1,2-dihydroxycyclooctane.
The synthesis of the catalysts of the formula (IV) is described in DE-A1-19647584. The palladaphosphacyclobutanes used generally have a dimeric structure. However, in the case of certain compounds (e.g. Y=acetylacetone, hexafluoroacetylacetone), monomeric, oligomeric or even polymeric structures can be present.
Preference is given to compounds of the formula (IV) in which
R1a and R2a are, independently of one another, hydrogen, methyl, ethyl, cyclopentyl, cyclohexyl, methoxy, ethoxy, fluorine, phenyl, tolyl or naphthyl;
R3a and R4a are, independently of one another, (C1-C4)-alkyl, (C5-C6)-cycloalkyl, substituted or unsubstituted C6-C10-aryl, or R3a and R4a together form an aliphatic ring having 5 or 6 carbon atoms;
R5a and R6a are, independently of one another, (C1-C4)-alkyl, (C5-C6)-cycloalkyl, phenyl, naphthyl, anthracenyl, which may be unsubstituted or substituted by from 1 to 3 CF3xe2x80x94, (C1-C4)-alkyl- or (C1-C4)-alkoxy groups;
and Y is acetate, propionate, benzoate, chloride, bromide, iodide, fluoride, sulfate, hydrogensulfate, nitrate, phosphate, triflate, tetrafluoroborate, tosylate, mesylate, acetylacetonate, hexafluoroacetylacetonate or pyrazolyl.
Particular preference is given to compounds in which
R1a and R2a are, independently of one another, hydrogen or methyl;
R3a and R4a are, independently of one another, methyl, ethyl or phenyl,
R5a and R6a are, independently of one another, phenyl, naphthyl, o-trifluoromethylphenyl, o-trifluoromethyl-p-tolyl, o-trifluoromethyl-p-methoxyphenyl, o-methoxyphenyl, o,p-dimethoxyphenyl, anthracenyl, tert-butyl, n-butyl, isopropyl, isobutyl, cyclohexyl or 1-methylcyclohexyl.
Very particular preference is given to the following compounds of the formula (IV):
trans-di-xcexc-acetato-bis[2-[bis(1,1 -dimethylethyl)phosphino]-2-methylpropyl-C,P]dipalladium(II),
trans-di-xcexc-acetato-bis[2-[1,1-dimethylethyl)phenylphosphino]-2-methylpropyl-C,P]dipalladium(II),
trans-di-xcexc-chloro-bis[2-[bis(1,1-dimethylethyl)phosphino]-2-methylpropyl-C,P]dipalladium(II),
trans-di-xcexc-chloro-bis[2-[1,1-dimethylethyl)-phenylphosphino]-2-methylpropyl-C,P]dipalladium(II),
trans-di-xcexc-bromo-bis-[2-[bis(1,1-dimethylethyl)phosphino]-2-methylpropyl-C,P]dipalladium(II) and
trans-di-xcexc-bromo-bis[2-[1,1-dimethylethyl)phenylphosphino]-2-methylpropyl-C,P]dipalladium(II).
During the catalysis cycle, the dimeric structure is broken up by means of bridge cleavage reactions with inorganic and organic nucleophiles so that the actual catalytically active species are likely to be the mononuclear complexes of the formula (VI) or (VII). 
The complexes of the formulae (VI) and (VII) are in equilibrium with the dimers actually used and are uncharged or anionic. The mononuclear complex of the formula (VI) may have further donor: ligands on the palladium atom.
The catalyst is advantageously used in a molar ratio to the compound of the formula (2) of from 10xe2x88x926 to 1, preferably from 10xe2x88x925 to 0.1, in particular from 10xe2x88x925 to 0.01.
The stability of the palladophosphacyclobutanes in solution can be increased by addition of alkali metal salts, alkaline earth metal salts and transition metal salts of transition groups VI to VIII. In particular, the addition of halides and pseudohalides of the metals mentioned in many cases gives a significant increase in yield and improvement in the operating life of the catalyst. Other suitable salts are ammonium halides, trialkylammonium and tetraalkylammonium salts and also corresponding phosphonium and arsonium salts.
As ionic halide, preference is given to using ammonium bromide, lithium bromide, sodium bromide, potassium bromide, tetrabutylphosphonium bromide, ammonium chloride, tetramethylammonium chloride, diethanolammonium chloride, lithium chloride, sodium chloride, potassium chloride, tetrabutylphosphonium chloride, ammonium iodide, lithium iodide, sodium iodide, potassium iodide and/or tetrabutylphosphonium iodide. Particular preference is given to lithium chloride.
The abovementioned salts are advantageously added in amounts of from 0 to 250 mol %, for example from 10 to 100 mol %, based on the compound of the formula (3).
Solvents used are generally inert organic solvents. Examples of well suited solvents are aromatic hydrocarbons such as toluene, xylenes, anisole, tetralin and aliphatic ethers such as tetrahydrofuran, dimethoxyethane, dioxane, tetrahydropyran and formaldehyde acetals. The amount of solvent is advantageously from 1 to 5000% by weight, preferably from 25 to 2000% by weight, particularly preferably from 50 to 1500% by weight, based on the weight of the compound of the formula (3).
The coupling of the invention generally proceeds at temperatures of from 20 to 200xc2x0C.; in many cases it has been found to be useful to work at temperatures of from 50 to 165xc2x0 C., preferably from 60 to 160xc2x0 C.
Bases used are, in particular, alkali metal alkoxides or alkaline earth metal alkoxides, alkali metal amides or alkaline earth metal amides, alkali metal acetates or alkaline earth metal acetates, alkali metal formates or alkaline earth metal formates, alkali metal propionates or alkaline earth metal propionates, alkali metal or alkaline earth metal carbonates, hydrogencarbonates, hydroxides or oxides and also aliphatic or aromatic amines. Particularly preferred bases are sodium carbonate or potassium carbonate, sodium hydroxide or potassium hydroxide, sodium tert-butoxide or potassium tert-butoxide and pyridine. The base is preferably used in an amount of from 0.5 to 5 equivalents, preferably from 0.8 to 4 equivalents and particularly preferably from 1 to 2 equivalents, based on the boron compound of the formula (3) used.